Project Summary According to the Center for Disease Control and Prevention, 40% of adults in the United States are obese and over 100 million Americans suffer from diabetes mellitus or prediabetes, most due to their obesity. Health care costs for individuals afflicted with diabetes are double compared to those whom don?t, so it is imperative for our society to fully characterize the link between obesity and metabolic disease. Since the discovery that adipose tissue is an active endocrine organ, intensive effort has focused on understanding the role of adipokine hormones in regulating metabolic homeostasis. One of these adipokines, adiponectin, has demonstrated great promise in promoting insulin sensitivity and in blunting obesity-associated inflammatory signaling. Previous studies have shown that bone marrow adipose tissue (BMAT) acts as a disproportionate source of circulating adiponectin through unknown mechanisms. During periods of postnatal development, calorie restriction, or irradiation, BMAT expands in volume and cell number from an undefined cellular progenitor. To test whether this precursor cell is a true preadipocyte or some other cell type, we have designed an elegant study where fluorescent protein reporter mice will be calorie restricted to potently stimulate BMAT expansion, then long bones will be sectioned and imaged. The results will show whether nascent BMAs arise from adiponectin-negative cells (preadipocytes) or an adiponectin-positive precursor population. The presence of adiponectin-positive precursors could indicate that dedifferentiation/delipidation of BMAs plays a role on the regulation of BMAT volume. To further understand the involvement of BMAT as a source of circulating adiponectin, we will investigate the mechanisms by which BMAT secretes disproportionately high amounts of adiponectin compared to other adipose depots. We have observed that the triacylglycerol (TAG) of bone marrow adipocytes (BMA) is significantly enriched in polyunsaturated fatty acid (PUFA) compared to white adipocytes, particularly for long- chain and highly-unsaturated PUFAs. Some PUFA species have been implicated in promoting adiponectin secretion by acting on various signaling elements. Based on these observations, we hypothesize that TAG PUFA mobilization via lipolysis leads to stimulation of adiponectin secretion in BMAT. To test this hypothesis, we will modify the TAG composition of cultured adipocytes via PUFA supplementation and/or knock out of key genes involved in fatty acid desaturation, elongation, and lipolysis. The results will then be confirmed in vivo utilizing transgenic mice to conditionally knock out these genes specifically in adipocytes. Combining these techniques with diet-induced obesity mouse models will reveal whether modulating BMAT secretion of adiponectin can correct the metabolic phenotype of obesity. Together, these studies will help characterize how BMAT secretes more adiponectin that other adipose depots, determine the potential of BMAT as a therapeutic target for metabolic disease, and elucidate the mechanism underlying BMAT expansion.